1. Field of the Invention
The present invention relates to thermostatically controlled temperature conditioning systems which circulate a fluid heat exchange medium to the areas to be conditioned and in particular to such systems that control the outlet temperature of the fluid medium inversely with outdoor temperature or other temperature reservoir producing an undesired loss or gain.
2. Description of the Prior Art
Temperature connditioning systems generally are used to provide a stable desirable temperature by overcoming heat transfer to or from a heat reservoir that is at a different temperature from the desired temperature. In a home temperature conditioning system the heat reservoir is the outdoor atmosphere and produces an undesired transfer of heat between the inside of the house and the outside of the house. The direction of transfer depends upon whether the heat reservoir (outside atmosphere) is warmer or cooler than the interior temperature. By anology this same situation exists in every circumstance where temperature control is applied. There is always some heat reservoir causing a transfer of heat, temperature control being applied to balance out the transfer. While the present invention applies broadly to most systems for this purpose, the description in the present application is directed primarily to a domestic central heating system as exemplary.
Modern domestic central heating systems commonly use hot water or hot air as a medium for extracting heat from the central heater and transporting it to the rooms to be heated. Also commonly, the central heater includes thermostatic controls that determine the temperature at which the heat exchange medium shall leave the central heater. This temperature is usually a fixed temperature that varies only by dropping when the central heater cannot replace heat as fast as it is lost. This temperature thus remains the same for an outdoor temperature of 60.degree. as it is for an outdoor temperature of 0.degree.. If it is set for a median point, then at 60.degree. with the room thermostat calling for 70.degree., the temperature in the rooms overshoots rapidly to 75.degree. or more and then waits a long time before the next cycle. These sudden surges of temperature change are uncomfortable and waste heat. At 0.degree. outside and the thermostat calling for 70.degree., the opposite problem occurs. Instead of too fast recovery, the recovery is too slow and the room temperature continues to fall even after the thermostat calls for heat resulting in large and uncomfortable temperature swings. Experience tells us that we can be quite comfortable at lower temperatures as long as they are stable. Thus a stable temperature at 68.degree. can be as comfortable as a varying temperature averaging 72.degree.. Recognizing this, large commercial establishments have for many years utilized expensive control systems to vary the thermostatic control of the heat exchange medium with changes in outdoor temperature. These systems today commonly use thermal expansion devices as temperature sensing elements. Thermal expansion devices for the most part are bulky, expensive and require a housing. Thermistors can be used, but have nonlinear characteristics which require compensation. This raises the cost, reduces reliability and increases replacement problems.
It has been known for some time that the forward voltage drop of a pn semiconductor junction varies linearly with temperature. This fact has already had numerous applications for temperature compensating of electronic circuits. It has had little recognition for general temperature sensing purposes. This may be due to the small voltage swings obtainable plus the much larger and obscuring nominal forward voltage drop.